Device-to-Device (D2D) or sidelink communications and cellular- or network-controlled D2D (sidelink) communications are expected to play an important role in the next generation wireless networks D2D technology can be used to provide diversity in space, time and frequency, and increase the robustness against fading and interference. User Equipment (UE) cooperation based on D2D is a technology that is receiving attention. With advances in D2D communications, UE cooperation is expected to play an important role in the future of wireless communication systems such as Long Term Evolution (LTE), also known as the fourth Generation (4G); enhanded LTE, eLTE advanced or fifth Generation (5G) and so on. Hereinafter by D2D communications is also meant sidelink communications.
One object with group transmission using D2D (“cooperative D2D”) may be a way to increase the coverage and user bit rate for example in the future high frequency 5G network. As mentioned, UE cooperation based on D2D, or, in other words, cooperative D2D, is gaining traction in the industry. A concept how group transmission using D2D can be performed was developed recently. According to the concept, the respective UEs in the group of UEs transmit synchronized as one antenna array in order to increase the UpLink (UL) coverage and bit rate. The UEs first transmit the data within the group using D2D methods and then transmit the same data jointly to the network. This is shown in FIG. 1 depicting 3 UEs 101, 102 and 103 respectively and a radio network node 104 acting as a base station or an eNB or a NB or an accept point etc. The UpLink and DownLink directions are also depicted.
When UE 101 wants to send data, it sends data to the other UEs 102 and 103 in the group of UEs 150 via D2D. The same data is transmitted synchronized to the radio network node 104. DL (control) data is received by UE 102, here acting as a coordinator or a relay of the group, for further relaying the DL (control) data to the other UEs 101 and 103.
The benefit from group transmission stems from the fact that the Signal-to-Interference-plus-Noise-Ratio (SINR) from the different UEs can be added, i.e. according to:
      S    ⁢                  ⁢    I    ⁢                  ⁢    N    ⁢                  ⁢          R      grouptx        =            ∑      u              Group        ⁢                                  ⁢        Size              ⁢          S      ⁢                          ⁢      I      ⁢                          ⁢      N      ⁢                          ⁢                        R          singletx                ⁡                  (          u          )                    
This is useful in low SINR scenarios or when the originating UE that wants to transmit data has a low SINR (or is even out of UL coverage) and the other UEs in the group have higher SINR. Hence, the SINR from the group transmission is typically considerably higher than for a specific UE within the group.
From a network node perspective, the group is seen as a single UE transmitting from different points. The group concept illustrated in FIG. 10 introduces a group IDendity (ID) which is used to hide from the network (node) that the group has several UEs. This simplifies the group transmission since the network (node) can reuse all DL legacy signaling (for single UEs) which may be relayed from the coordinator UE to the other group members or be received directly from the network node (e.g. eNodeB) by the UEs.
An illustration of a group of UEs (UE1, UE2, UE3) sharing a common queue with data for UL transmission is depicted in FIG. 2. The data from UE2 and UE3 have been distributed by D2D communication within the group before the UL transmissions to the network node. All UEs have identical group HARQ buffers. FIG. 2 is self-explanatory.
Further advantage of the group transmission is that also UEs that are out of UL coverage to the base station contribute to an increased SINR and hence UL throughput for the group.
With group transmission, the basic idea is that the network node or base station sees the group as one single UE and receives the same data synchronized from all UEs in the group. This means that all UEs must have the same data in their group transmission Hybrid Automatic Repeat ReQuest (HARQ) buffers and their transmissions completely synchronized. Since the UL transmissions rely on HARQ feedback, in the form of an ACKnowledgment (ACK) or Non-ACK (NACK), reception of the feedback need to be done by all UEs and also interpreted the same way by all UEs.
FIG. 3 illustrates two ways or methods 300A resp. 300B to transmit HARQ feedback.
In the first approach 300A, if the HARQ feedback (ACK/NACK) is transmitted via the coordinator, i.e. relayed, this leads to additional delays and the HARQ protocol would need to be altered in order to cater for the group transmissions (i.e. separate protocol for group transmissions would be needed). However, this approach would allow an increase of the reliability of the D2D link (e.g. using harder or different coding etc.) which would reduce the total error rate.
In the second approach 300B, the HARQ feedback (ACK/NACK) is transmitted to the individual UEs separately, there would be a risk that the HARQ feedback is interpreted differently at the different UEs causing reduced reliability even though, there would be no extra delays. This approach to transmit HARQ feedback is used in the embodiments that will be described in greater details in the detailed description part.
It should be noted that when the UEs HARQ buffers becomes unsynchronized, not only does it no longer increase the SINR of the group transmission, it even makes the performance worse than if it had not been a part of the group at all since it will generate interference instead. Hence, maintaining synchronization of the UL transmissions is of importance for efficient UL group transmissions.
An example that will lead to unsynchronized transmissions is illustrated in FIG. 4. A first UE (UE1) interprets a received ACK as a NACK. Then this first UE will attempt to retransmit the packet in a coming Transmission Time Interval (TTI). At the same time the other UEs in the group will transmit a different packet in this TTI. For the receiving network node or eNodeB, the retransmission from the first UE (UE1) will be received as noise or interference and make the reception of the correct signal from the other UEs more difficult. Hence the reception of the desired signal will be impaired by interference caused by the first UEs erroneous retransmission. This error will continue in subsequent TTIs as the first UE is unsynchronized with the other UEs in the group. Note that the same problem will occur if a NACK would be interpreted as an ACK. An illustration of the problem when a NACK is miss-interpreted as an ACK by one UE in the group is depicted in FIG. 4.
The possibility for a UE's HARQ buffer to become unsynchronized increases with the size of the group and may quickly become disturbing for the performance of the group transmission.
Autonomous HARQ retransmission may be another option for UL group transmission. UEs participating in group transmission always, in this Autonomous HARQ retransmission scenario, perform retransmission a given number of transmission attempts, regardless of the receiver's reception results. The receiver does not need to provide HARQ feedback. However, an issue is that autonomous retransmissions may increase the Radio Link Control (RLC) layer retransmissions and lead to degraded UL capacity.